1. Field of the Invention
The present invention is directed to a method and catalyst system for producing aromatic carbonates and, more specifically, to a method and catalyst system for producing diaryl carbonates through the carbonylation of aromatic hydroxy compounds.
2. Discussion of Related Art
Aromatic carbonates find utility, inter alia, as intermediates in the preparation of polycarbonates. For example, a popular method of polycarbonate preparation is the melt transesterification of aromatic carbonates with bisphenols. This method has been shown to be environmentally superior to previously used methods which employed phosgene, a toxic gas, as a reagent and chlorinated aliphatic hydrocarbons, such as methylene chloride, as solvents.
Various methods for preparing aromatic carbonates have been previously described in the literature and/or utilized by industry. A method that has enjoyed substantial popularity in the literature involves the direct carbonylation of aromatic hydroxy compounds with carbon monoxide and oxygen. In general, practitioners have found that the carbonylation reaction requires a rather complex catalyst system. For example, in U.S. Pat. No. 4,187,242, which is assigned to the assignee of the present invention, Chalk reports that a carbonylation catalyst system should contain a Group VIII B metal, such as ruthenium, rhodium, palladium, osmium, iridium, platinum, or a complex thereof. Further refinements to the carbonylation reaction include the identification of organic co-catalysts, such as terpyridines, phenanthrolines, quinolines and isoquinolines in U.S. Pat. No. 5,284,964 and the use of certain halide compounds, such as quaternary ammonium or phosphonium halides in U.S. Pat. No. 5,399,734, both patents also being assigned to the assignee of the present invention.
The economics of the carbonylation process is strongly dependent on the number of moles of aromatic carbonate produced per mole of Group VIII B metal utilized (i.e. "catalyst turnover"). Consequently, much work has been directed to the identification of efficacious catalyst combinations that increase catalyst turnover. In U.S. Pat. No. 5,231,210, which is also assigned to the present assignee, Joyce et al. report the use of a cobalt pentadentate complex as an inorganic co-catalyst ("IOCC"). In U.S. Pat. No. 5,498,789, Takagi et al. report the use of lead as an IOCC. In U.S. Pat. No. 5,543,547, Iwane et al. report the use of trivalent cerium as an IOCC. In U.S. Pat. No. 5,726,340, Takagi et al. report the use of lead and cobalt as a binary IOCC system.
Carbonylation catalyst literature lauds the effectiveness of bromide compounds as a halide source in the catalyst system. For example, in the aforementioned U.S. Pat. No. 5,543,547, Iwane et al. state the traditional understanding that bromide sources are the preferred halide sources and that chloride is known to exhibit low activity. While it is true that bromide has historically exhibited higher activity, there are drawbacks to using bromide in the carbonylation reaction. Initially, it is worth noting that onium bromide compounds are typically expensive compared to, e.g., onium chloride compounds. Furthermore, when used to carbonylate phenol, bromide ion is consumed in the process forming undesirable brominated byproducts, such as 2- and 4-bromophenols and bromo diphenylcarbonate. These byproducts must typically be recovered and recycled, further adding to the investment and operating cost of the process. However, due to their comparatively low activity, onium chloride compounds have not traditionally been considered an economically viable alternative to onium bromide compounds.
Unfortunately, the literature is not instructive regarding the role of many catalyst components in the carbonylation reaction (i.e. the reaction mechanism). Accordingly, meaningful guidance regarding the identification of effective combinations of catalyst system components is cursory at best. In this regard, periodic table groupings have failed to provide guidance in identifying additional IOCC's. For example, U.S. Pat. No. 5,856,554 provides a general listing of possible IOCC candidates, yet further analysis has revealed that many of the members (and combinations of members) of the recited groups (i.e., Groups IV B and V B) do not effectively catalyze the carbonylation reaction. Therefore, due to the lack of guidance in the literature, the identification of effective carbonylation catalyst systems has become a serendipitous exercise.
As the demand for high performance plastics has continued to grow, new and improved methods of providing product more economically are needed to supply the market. In this context, various processes and catalyst systems are constantly being evaluated; however, the identities of improved and/or additional effective catalyst systems for these processes continue to elude the industry. Consequently, a long felt, yet unsatisfied need exists for new and improved methods and catalyst systems for producing aromatic carbonates and the like.